Device-to-Device (D2D) communication refers to direct communication between devices. In D2D communication, data to be transmitted from a first device to a second device is not relayed via any cellular network node. Some examples of D2D communication of the prior art is used in standards like Bluetooth, and WLAN (e.g. IEEE 802.11).
Device-to-device communication may be applicable in various scenarios. One scenario is when a cellular radio access network is present, and able to set up a cellular connection between two devices. D2D communication may be a complement to the cellular communication in such scenarios.
There may be situations when D2D communication provides better performance (better signal quality, higher bit rate, lower latency, etc.) than cellular communication. This may be due to proximity between the devices and/or specific signaling gain of the D2D protocol.
In some situations, the network may have constraints (e.g. due to being heavily loaded) resulting in that a service cannot be provided at all using a network connection. Then, D2D communication would be an alternative.
There may also be situations when D2D communication is preferred by the user of a device (e.g. due to billing costs).
D2D communication may improve spectrum efficiency and reduce the network load for the cellular network, since the D2D connection typically, but not necessarily, uses another spectrum range (e.g. an unlicensed spectrum) than the cellular network (typically licensed spectrum). Furthermore, since cellular communication uses an uplink-downlink pair for each of the two devices while a D2D connection would only use one link pair, spectrum efficiency is improved even if the D2D connection would use cellular spectrum resources. This is also true for network assisted D2D communication, where most of the data is transmitted over the D2D connection and only a small amount of information is transmitted over the network links.
D2D communication may be ad hoc or may be network assisted. For example, a cellular network may assist a D2D connection by establishing security of the D2D link(s) and/or partly or fully controlling the setup of the D2D connection(s) (e.g. device/peer discovery and resource allocation). A cellular network may also assist D2D communication by controlling the interference environment. For example, if using licensed operator's spectrum for the D2D communication, higher reliability can be provided than when operating in unlicensed spectrum. To assist the D2D connection, the network may also provide synchronization and/or partial or full Radio Resource Management (RRM). RRM may e.g. comprise time and/or frequency resource allocation for the D2D communication.
Mobility management becomes more complicated in scenarios with D2D communication. For example, handover of the cellular communication related to a device that moves from one cell coverage area to another cell coverage area typically needs to consider how to handle network assisted D2D communication that the device is involved in.
Traditionally, both devices involved in a D2D link are handed over jointly, thereby also handing over the control of the D2D communication; see, for example, WO 2011/109027 A1.
A problem with this approach is that it is complicated and typically requires introduction of network signaling to handle the joint handover. There is a risk that the network control of the D2D communication is lost during the handover. Furthermore, if one of the devices need to do the handover to not lose its cellular communication link while the other device will lose its cellular communication link if it does the handover (i.e. if the two devices are in different cell areas and not in an overlapping cell region), the joint handover approach may be fatal.
Thus, there is a need for improved methods and arrangements for handover in network assisted device-to-device communication scenarios.